FIG. 3 is a block diagram of conventional receiver 1 disclosed in Patent Document 1. Receiver 1 includes first receiving section 2 and second receiving section 3. First receiving section 2 includes first antenna terminal 4, first radio-frequency (RF) amplifier 5 having an input port connected to first antenna terminal 4, first mixer 6 having a first input port connected to an output port of first RF amplifier 5, and first local oscillator 7 connected to a second input port of first mixer 6. Second receiving section 3 includes second antenna terminal 8, second RF amplifier 9 having an input port connected to second antenna terminal 8, second mixer 110 having a first input port connected to an output port of second RF amplifier 9, and second local oscillator 111 connected to a second input port of second mixer 110.
When a receiving condition is not better than a predetermined level due to an influence of fading during moving, receiver 1 is put into a diversity receiving mode in which both of first receiving section 2 and second receiving section 3 receive a signal. On the other hand, when the receiving condition becomes better than the predetermined level stably, receiver 1 is put into a single receiving mode in which one of first receiving section 2 and second receiving section 3 exclusively receives the signal. This operation provides receiver 1 with high sensitivity even in a poor receiving condition due to the moving. Further, when the receiving condition becomes better, receiver 1 can turn off one of the receiving sections, accordingly reducing power consumption.
In the single receiving mode in which first receiving section 2 of receiver 1 is activated, first mixer 6 heterodynes a signal having a desired frequency supplied from first antenna terminal 4 to a signal having an intermediate frequency with using an oscillation signal output from first local oscillator 7. First antenna terminal 4 may receive an interference signal having an adjacent frequency within a predetermined range from a sum of or a difference between a frequency of the oscillation signal and the desired frequency. In this case, If first RF amplifier 5 is not sufficiently isolated from first local oscillator 7, first RF amplifier 5 mixes the interference signal with a noise having the frequency of the oscillation signal emitted from first local oscillator 7 and generates a distortion signal having a frequency close to the desired frequency, hence having a receiving quality deteriorate.
Receiver 1 has been recently demanded to have a small size. Therefore, first receiving section 2 and second receiving section 3 are mounted onto a small multilayer circuit board as a single module, or are integrated into a small semiconductor package. These arrangements cause first RF amplifier 5 to be hardly isolated from first local oscillator 7, accordingly causing the above problem to occur.
FIG. 4 is a block diagram of another conventional receiver 101 disclosed in Patent Document 1. Receiver 101 includes first receiving section 102 and second receiving section 103. First receiving section 102 includes first antenna terminal 104, first RF amplifier 105 having an input port connected to first antenna terminal 104, first mixer 106 having a first input port connected to an output port of first RF amplifier 105, and first local oscillator 107 connected to a second input port of first mixer 106. Second receiving section 103 includes second antenna terminal 108, second RF amplifier 109 having an input port connected to second antenna terminal 108, second mixer 10 having a first input port connected to an output port of second RF amplifier 109, and second local oscillator 11 connected to a second input port of second mixer 10 and the second input port of first mixer 106.
In a diversity receiving mode in which both of first receiving section 102 and second receiving section 103 receive a signal, first mixer 106 heterodynes a signal output from RF amplifier 105 with using an oscillation signal output from first local oscillator 107. Second mixer 110 heterodynes a signal output from RF amplifier 109 with using the oscillation signal output from first local oscillator 107. Since second receiving section 103 uses first local oscillator 107 commonly with receiving section 102, receiver 101 can turn off second local oscillator 111 to reduce power consumption in the diversity receiving mode.
In receiver 101, first antenna 104 receives an interference signal having an adjacent frequency within a predetermined range from a sum of or a difference between a frequency of the oscillation signal and the desired frequency. In this case, if first RF amplifier 105 is not isolated sufficiently from first local oscillator 107, first RF amplifier 105 mixes the interference signal with a noise having the frequency of the oscillation signal emitted from first local oscillator 107, and generates a distortion signal having a frequency close to the desired frequency, hence having a receiving quality deteriorate.
Receiver 101 has been recently demanded to have a small size. Therefore, first receiving section 102 and second receiving section 103 are mounted onto a small multilayer circuit board as a single module, or are integrated into a small semiconductor package. These arrangements cause first RF amplifier 105 to be hardly isolated from first local oscillator 107, accordingly causing the above problem.    Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-130279